Field of the Invention
The field of the present invention is flexible seals. More particularly, the invention relates to a bi-directional flexible seal for use in drum type heat exchangers.
In large-scale power generating facilities, it is generally desirable to pre-heat the air provided for combustion, to improve the thermodynamic efficiency of the power generation and to reduce fuel consumption. One type of air preheater (the Ljungstrom type) includes a rotating drum having heat exchange surfaces or plates attached to and continuously rotating with the drum. As the drum rotates, the heat exchange plates move from a combustion air flow side to an exhaust gas flow side and then back to the combustion air flow side. In this manner, the heat exchange plates of the drum continuously transfer heat from the hot exhaust gases passing over the plates to the combustion air, thereby raising the temperature of the combustion air and improving efficiency.
In order to prevent mixing of the exhaust gases with the combustion air during the heat exchange process, a sector plate is provided on each flat or radial side of the drum and a circumferential ring surrounds the cylindrical outside of the drum within the air preheater housing. The drum has diaphragms projecting radially outward to the ring from the drum driveshaft. As each diaphragm passes from the combustion air section to the exhaust gas section of the air preheater, the diaphragm is sealed against the sector plate to prevent mixing of the exhaust and combustion air flows. Circumferential seals and shaft seals are generally also provided.
Various radial seals for intermittently sealing the diaphragms as they pass between the sector plates have been known in the past. These known seals have met with varying degrees of success. In practice, it has often proven difficult to obtain consistent performance from radial seals. One major difficulty encountered is the dimensional variations within the drum and preheater caused by temperature variations. Although the seals are necessarily installed in the preheater when it is off-line and at room temperature, once in operation, localized drum temperatures rise significantly. Since the drum is large (typically from 12 to 36 feet in diameter), the coefficient of thermal expansion of the preferred construction metals can produce considerable dimensional shifts or distortion resulting in inadequate sealing between the diaphragms and the sector plates.
In addition, the repeated intermittent sealing engagement of the sealing surfaces against the sector plates results in a wearing down of the surfaces which can also lead to decreased sealing efficiency. Wear problems can become especially severe if the sealing surfaces are engaged too closely to the sector plate (overengagement). This condition resulting from improper alignment of the seals also creates excessive seal stresses which can lead to early seal failure.
Drum type air preheaters may be oriented with the axis of rotation of the drum being either vertical (i.e. in a "carousel" orientation) or horizontal (i.e. a ferris wheel orientation) with the horizontal access ("ferris wheel" orientation). Due in part to the usual imbalance in the drum, when the air preheater is shut down and the drive motor disengaged, the drum may rotate forward (i.e. in its driven direction) or backward by a small amount. This continued "free wheeling" rotation in a forward direction is ordinarily acceptable. However, if the wheel rotates backwards or in the reverse direction, certain seals may be damaged since they are designed and intended to engage the sector plates only when moving in the forward direction.